The present invention relates to a spark plug and a method of producing the spark plug.
Conventionally, the spark plug has an electrode having a chip at an end of the electrode. The chip is so welded as to form an igniter. It is Pt (platinum) that is used for a principal component of the chip of the electrode, to thereby improve spark durability. Recently, in order to further improve the spark durability, as the case maybe, Ir (iridium) replaces Pt for the main component of the chip of the electrode of the spark plug.
The spark plug having the above one of Pt and Ir as the material of the igniter is used for a gas engine. For example, the gas engine is the one referred to as a cogeneration gas engine which utilizes both emission heat and combustion heat. In this case, during a combustion process of a mixture in a combustion chamber of the internal combustion engine, the igniter of the spark plug is likely to be subjected to a cooling-and-heating cycle. More specifically, the cooling-and-heating cycle causes a quick cooling during a mixture intake process, and a quick heating during a mixture combustion process. Such cooling-and-heating cycle is more likely to occur to a lean burn engine which is designed to reduce NOx and the like contained in emission gas.
The cooling-and-heating cycles (heavy duty) repeatedly applied to the igniter causes the igniter to have its metal surface peeled. The thus peeled metal piece is melted by a discharge, to thereby cause a sweat (a phenomenon in which the melted metal piece jumps, and then re-adheres). The peel and the sweat may cause the metal pieces to be accumulated across a spark discharge gap, to thereby cause a bridging. This is likely to cause an ignition failure attributable to a short gap. Especially, many of the spark plugs for the gas engine are likely to cause the bridging and the like since the gap of the gas engine is so small as to obtain a lower discharge voltage.